A method for communication includes, receiving in a first communication interface input frames, which include data symbols derived by encoding respective characters, and one or more synchronization symbols distinguishable from the data symbols. The characters are recovered from the data symbols, and transmitted to a second communication interface by mapping the characters into communication frames and discarding the synchronization symbols, wherein a protocol for delivering the characters supports a mapping scheme for delivering the characters at a first data rate, and mapping the characters includes mapping the characters so as to deliver the characters at a second data rate lower than the first data rate. The characters are extracted from the communication frames in the second communication interface by synchronizing to the mapped input frames independently of the synchronization symbols. The input frames are reconstructed to including the data symbols and the synchronization symbols, by re-encoding the extracted characters.